XenForo & Embed media settings.

I'm jsut curious where is placed css which will give me the chance 2 edit/add my own bgs/layers etc for embed media inputs, such as YouTube etc.

E.g: When I do any input (youtube for example) i got an embded video, however it's not enough. Need 2 add a custom background for these embed codes, 2 make it look like you place a video in 2 the custom box, like we do with Quotes. You place a text in 2 the diff box which u can animate as u like.